1. Field of the Invention
The present invention relates to a balun, and more particularly, to a balun capable of improving a matching impedance characteristic, a signal pass characteristic and an isolation characteristic between balanced terminals by using a transmission line and a resonant circuit.
2. Description of the Related Art
A balun is a circuit or a device that converts a balanced signal into an unbalanced signal or converts an unbalanced signal into a balanced signal. The balun may be implemented by using a combination of transmission lines or a lumped element.
FIGS. 1A and 1B are a diagram and an equivalent circuit diagram of a related art balun circuit.
Referring to FIG. 1A, the related art balun circuit is implemented by using a transmission line 110 having an electrical length of λ/4 and a transmission line 120 having an electrical length of 3 λ/4.
The balun circuit of FIG. 1A may be represented by an equivalent circuit of FIG. 1B. In the equivalent circuit of FIG. 1B, a first inductor 121 and a second capacitor 122 formed between an input terminal 101 and a first output terminal 102 may operate as a low-pass filter using a center frequency of the balun circuit as a cutoff frequency. A first capacitor 131 and a second inductor 132 formed between the input terminal 101 and a second output terminal 103 may operate as a high-pass filter.
The balun circuit configured as above cannot attain excellence in a reflection loss characteristic of an unbalanced terminal, a signal pass characteristic between the input terminal and the output terminal, and a signal isolation characteristic between two balanced terminals.